Receptors are key molecules that recognize the target molecule on the extracellular side of the cell and initiate a series of downstream intracellular processes. Bacterial receptors have similar domain structures as many mammalian hormone receptors in that they have ligand binding components on the extracellular side, transmembrane components which span the membrane, and cytoplasmic components for effector binding inside the cell. Two receptor systems will be studied using X-ray crystallographic methods: (1) Bacterial aspartate receptor for chemotaxis in Salmonella and E.coli. (2) Two transport receptors from Salmonella, one for transport of the amino acid histidine and the other for transport of lysine/arginine/ornithine. Our overall objective is to determine the three-dimensional structure of the components and domains of these bacterial receptors. This would enable us to understand the signal transduction mechanism of chemotaxis and amino acid transport including the ligand recognition, signal transduction through the transmembrane and adaptation at the cytoplasmic domain. We have so far crystallized three proteins: the ligand binding domain of aspartate receptor and two amino acid receptors: one for histidine transport and the other for lysine/arginine/ ornithine transport.